feat: add IN [...] coord filter to sum_over

[jc-macdonald]

This pull request adds support for filtering the coordinates included in sum_over expressions using an optional IN [...] clause. This allows users to specify a subset of axis coordinates to sum over, rather than always summing over all coordinates. The implementation includes robust error checking for invalid, empty, unknown, or duplicate coordinates in the filter. Comprehensive tests have been added to ensure correct behavior and error handling.

Enhancements to sum_over syntax:

• Updated the _SUM_OVER_RE regular expression in src/op_system/specs.py to support an optional IN [...] filter for specifying a subset of coordinates to sum over.
• Added the _apply_coord_filter helper function to validate and apply the coordinate filter, raising errors for empty, unknown, or duplicate coordinates.
• Modified the _expand_sum_over logic to use the filtered coordinate list when expanding sums, ensuring only the specified coordinates are included.

Testing and validation:

• Added multiple tests in tests/op_system/test_op_system_specs.py to verify correct expansion and error handling for the new sum_over IN [...] syntax, including cases for valid filters, empty filters, unknown coordinates, and duplicates.
• Added an end-to-end test in tests/op_system/test_op_system_compile.py to ensure that the filtered sum compiles and evaluates correctly.

[pearsonca]

Minor notes for future work

[pearsonca]

note for future work: desire to avoid needing regular expressions. I think the ideal here is figuring out how to rely on the equation / symbol parsing libs.

[pearsonca]

what's error like for, say, a missing close ]? we should figure out (perhaps not in this PR) how to handle that sort of problem gracefully and with useful feedback.